Elecmon is the rookie level partner to Jeri Katou. But during their initial meeting, he was at the champion level as Leomon. Courageous and noble and a firm believer that every living being has their own destiny to live out, Leomon is a master of the secret styles of martial arts. From his fist, he can summon a powerful flaming projectile known as the Fist of the Beast King.

Before Bio-Emerging to the Real World, Leomon only knew of endless conflict. At first, he was a bit frightened by Jeri's affection and insistence that he's her partner. But he grew to care for her, becoming Jeri's partner for real after his near-death experience with Zhuqiaomon.

Despite Jeri's doubts of being worthy enough to be his Tamer, Leomon had much confidence in her. He once commented that she was such a little thing but she had a lion's heart, although this line is a dub exclusive. But things went downhill during the fierce battle with Beelzemon, as Leomon seized his arm to stop him from killing Kyubimon. Leomon informed the ranting Beelzemon that to simply have power did not make him strong – but Beelzemon retaliated by impaling Leomon through the chest with his claws. Leomon died and Beelzemon absorbed his data, with the aftermath being Jeri's depression that made her an ideal subject for the D-Reaper's research. His final words to Jeri were "a part of me will always be with you, remember you have a lion's heart." In the Japanese version he simply made a remark about fate, something he speaks of frequently in this version of the show. Jeri eventually overcame her sadness and remembering Leomon's words gave her the confidence to stand up to the D-Reaper. Unlike Angemon's death in Adventure and Wormmon's death (then later rebirth) in 02, this is the only occasion a DigiDestined's partner dies with no Digi Egg to bring him back to life, because Beelzemon absorbed his data. His spirit did seem to return to help at times: Beelzemon Blast Mode attempted to free Jeri from the Kernel Sphere and he finally broke in because suddenly he was able to call upon Leomon's Fist of the Beast King attack to break the Sphere. When he did this an image of Leomon appeared over him almost like his spirit was lending a hand. Also when Jeri started to regain her confidence she asked him a question and an image of him appeared on her Digivice and nodded at her.

In Dimensions, his data was extracted from Impmon's by a machine created by the Monster Makers in Book One. This allowed him to be reunited with Jeri and for her to once more fulfill her role as a tamer. For a while, his large size proved slightly problematic. During Book Two, he accompanies Jeri, Henry, and Terriermon to the Digidestined universe. In Book Three, he dedigivolves to his lower stage of Elecmon (but, unlike most, the markings on him are yellow instead of blue because he is a vaccine type instead of data type) after a battle with Barbamon. After this point, he normally remains in his rookie level.

In Book Four, Elecmon accompanied Jeri and some of the other tamers to New York for the conference. During a battle there, the red rookie and girl Biomerged together into SaberLeomon for the first time. Elecmon continues to be a valuable ally during other battles and even tries to talk Jeri out of harming Ryder while she was under Chiyo's control. He also tried to protect his partner when an angry crowd turned on the tamers after Rika was framed for the murder of the Prime Minister, though his actions only further upset the people.